Slap-happy azobenzenes

If anyone came up to me and asked me, "Hey Excimer, what's your favorite functional group?" I would be very puzzled how they knew who I was based on my blog handle. But after questioning them NSA-style and maybe a little executive branch-approved torture (thanks PATRIOT Act!), I'd let them know that my favorite functional group is, in fact, the azobenzene group. (Then I would probably eliminate them.)

Azobenzenes are strange duckies. They are brilliant chromophores- azobenzene by itself is a strikingly bright orange, and functionalized azobenzenes, whose colors can range from yellow to blood red, take up a portion of the so-called FD&C (food, drug and cosmetics) dyes. Perhaps more striking than azobenzene's pretty color is their ability to photoisomerize. Azobenzenes can cis/trans isomerize in the presence of light. Under ultraviolet light, trans-azobenzene isomerizes to cis-azobenzene; under blue light, the reverse occurs. cis-azobenzene can thermally relax to the trans-isomer in the dark as well:


This strange property of azobenzenes makes it a curious material for various applications, which are numerous and I can't find my favorite examples at the moment (maybe for another post). The Zink group at UCLA exploited this isomerization in their latest J. Phys. Chem. C paper on ASAP right now. In it, azobenzene-functionalized mesoporous silica nanoparticles (with 2 nm pores) were embedded with the dye molecules Rhodamine 6G or Coumarin 540A. To keep the dye molecules in the pores, the pores were capped with 1 nm-large dendrimer-functionalized azobenzene. Irradiation at 457 nm causes both cis- and trans-azobenzene to isomerize, flopping back and forth between states and essentially slapping the dye molecules out of the pores.


I like to think of it as one of those obstacle courses on American Gladiators where you, as the dye molecule, have a bunch of padded bats (the azobenzenes) flying at you while you're trying to escape, and then Ice tries to take you out with her tennis ball machine gun. Or something.

It's a crazy idea, but it actually works, based on luminescence spectra. Only when the system is irradiated with light that can isomerize azobenzenes do the dye molecules actually leave the pores they are trapped in. This is yet another example of the applicability of azobenzenes in so-called molecular machines- there are lots of really awesome similar ideas out there in the literature, and, God willing, some of them might actually end up being useful; and, God willing, I'll find the time to find those examples I like so much and fawn over them here.